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‘TIS THE SEASON FOR GREETING NEW series. But we must also bid adieu to an old favorite: Clarissa Explains It All, which has its final outing Saturday on Nickelodeon (Oct. 1,8 p.m. ET). Our teen heroine is getting ready to graduate and step out into the world by herself. What apostasy! The show’s star, Melissa Joan Hart, is only 18. TV actors rarely graduate from high school until they’re in their 30s. But such verisimilitude is typical of this sitcom. Vivacious, loopy and bright—this has been one happening show for kids.